1. Field of the Invention
This invention relates to a composite sheet for use in thermoforming shaped articles. More particularly, the invention concerns such a composite sheet which comprises a layer of thermoplastic polymer and a specific stitchbonded nonwoven elastic fabric that can be thermoformed into more uniform, lighter weight objects than could have been thermoformed with the same layer of thermoplastic plastic, were the fabric layer omitted.
2. Description of the Prior Art
Conventional thermoforming processes are described by Lubin, "Handbook of Composites", Van Nostrand Reinhold Company, 1982, pages 366-367. Such thermoforming generally involves the sequential steps of (a) heating a thermoplastic sheet until it softens, (b) forming the softened sheet under the influence of gravity, pressure and/or vacuum in a mold and (c) allowing the formed sheet to cool and harden. When thermoforming is used to mold a sheet into a large article, such as the interior surface of a bathtub, a shower stall, sink, cooler, canoe hull, or the like, the thermoplastic sheet must undergo a large increase in area, often by a factor of three or more, along with a corresponding decrease in thickness. Depending on the area change and the design of the mold, among other things, there are often undesired thin spots in the molded article. During thermoforming, regions of the thermoplastic sheet subjected to large area changes or rapid contour changes, such as the corners and edges of the shaped article, often suffer from excessive and nonuniform thinning, which in turn leads to lower impact resistance, lower flexural strength and greater fragility. To compensate for such excessive, non-uniform thinning, the molder usually provides extra reinforcement to such regions by using a much thicker thermoplastic sheet for the thermoforming. This results in a heavier and more expensive molded article. Accordingly, it is an object of this invention to provide a thermoformable sheet material that will resist undesired thinning during such thermoforming molding operations.
Composite sheets comprising thermoplastic sheets laminated to other layers of plastic sheets or films are known for use in thermoforming processes. For example, U.S. Pat. No. 4,415,519 (Strassel), U.S. Pat. No. 4,594,290 (Fischer et al), European Patent Application 87103935.0 (Besozzi), and Japanese Patent Application Publication 63-111050 (Makihata) disclose such composite sheets. Composite sheets comprising a thermoplastic layer and a fabric, also are known for use in thermoforming, as for example from Japanese Patent Application Publication 63-162238 (Iwasaki), and U.S. Pat. No. 4,298,643 (Miyagawa et al). Miyagawa et al discloses the use of various types of yarns (e.g., textured, spandex, conventional) in a particular knitted fabric that is laminated to a thin thermoplastic sheet to form a composite sheet suitable for thermoforming.
Not related to thermoplastic molding is the present inventor's U.S. Pat. No. 4,773,238, which discloses an elastic stitchbonded nonwoven fabric that is useful as a dust cloth, industrial wipe, insulating fabric, and the like. An elastic stitching thread provides the fabric with certain elastic properties. Various elastic threads and substrate compositions are disclosed.